Method and Apparatus for Forming a Thin Lamina

ABSTRACT

A method for producing a lamina from a donor body includes implanting the donor body with an ion dosage and separably contacting the donor body with a susceptor assembly, where the donor body and the susceptor assembly are in direct contact. A lamina is exfoliated from the donor body, and a deforming force is applied to the lamina or to the donor body to separate the lamina from the donor body.

RELATED APPLICATIONS

This application is a continuation in part to Murali et al., U.S. patentapplication Ser. No. 12/980,424, “A Method to Form a Device byConstructing a Support Element on a Thin Semiconductor Lamina” filed onDec. 29, 2010, owned by the assignee of the present application, andhereby incorporated by reference. Pursuant to 35 U.S.C. §119 (e), thisapplication claims priority to the filing dates of: U.S. ProvisionalPatent Application Ser. No. 61/510,477, “Detection Methods inExfoliation of Lamina,” filed on Jul. 21, 2011; U.S. Provisional PatentApplication Ser. No. 61/510,476, “Support Apparatus and Methods ForProduction of Silicon Lamina,” filed on Jul. 21, 2011; U.S. ProvisionalPatent Application Ser. No. 61/510,478, “Ion Implantation andExfoliation Methods,” filed on Jul. 21, 2011; and U.S. ProvisionalPatent Application Ser. No. 61/510,475, “Apparatus and Methods forProduction of Silicon Lamina,” filed on Jul. 21, 2011; the disclosuresof which applications are herein incorporated by reference. Thisapplication is also related to Kell et al., U.S. patent application Ser.No. ______ (Attorney Docket TwinP064a/TCA-091y), entitled “A Method andApparatus for Forming a Thin Lamina” filed on even date herewith, whichis hereby incorporated by reference.

BACKGROUND OF THE INVENTION

A conventional prior art photovoltaic cell includes a p-n diode; anexample is shown in FIG. 1. A depletion zone forms at the p-n junction,creating an electric field. Incident photons (incident light isindicated by arrows) will knock electrons from the valence band to theconduction band, creating free electron-hole pairs. Within the electricfield at the p-n junction, electrons tend to migrate toward the n regionof the diode, while holes migrate toward the p region, resulting incurrent, called photocurrent. Typically the dopant concentration of oneregion will be higher than that of the other, so the junction is eithera p+/n− junction (as shown in FIG. 1) or a n+/p− junction. The morelightly doped region is known as the base of the photovoltaic cell,while the more heavily doped region, of opposite conductivity type, isknown as the emitter. Most carriers are generated within the base, andit is typically the thickest portion of the cell. The base and emittertogether form the active region of the cell.

Ion implantation is a known method for forming a cleave plane in asemiconductor material to form lamina utilized in photovoltaic cells.The ion implantation and exfoliation steps in these methods may have asignificant effect on the quality of the lamina produced. It isdesirable to improve the methods and apparatus for producing lamina.

SUMMARY OF THE INVENTION

A method for producing a lamina from a donor body includes implantingthe donor body with an ion dosage and separably contacting the donorbody with a susceptor assembly, where the donor body and the susceptorassembly are in direct contact. A lamina is exfoliated from the donorbody, and a deforming force is applied to the lamina or to the donorbody to separate the lamina from the donor body.

BRIEF DESCRIPTION OF THE DRAWINGS

Each of the aspects and embodiments of the invention described hereincan be used alone or in combination with one another. The aspects andembodiments will now be described with reference to the attacheddrawings.

FIG. 1 is a cross-sectional view of a prior art photovoltaic cell.

FIG. 2A through 2D are cross-sectional views showing stages in theformation of the photovoltaic device of Sivaram et al., U.S. patentapplication Ser. No. 12/026,530.

FIG. 3 is a flow chart showing steps of an exemplary method according toaspects of the present invention.

FIG. 4A and 4B are cross sectional views showing stages of laminaformation according to embodiments of the present invention.

FIG. 5A and 5B are cross sectional views showing lamina separationaccording to embodiments of the present invention.

FIG. 6A and 6B are cross sectional views showing lamina separationaccording to embodiments of the present invention.

FIG. 7A through 7C show cross sectional views of stages in the formationof a photovoltaic device having a constructed metal support element.

FIG. 8A and 8B are perspective cross sectional and perspective top viewdiagrams of an exemplary susceptor assembly of this invention.

FIG. 9A and 9B are top views showing embodiments of susceptor plates ofthis invention.

FIG. 10A and 10B are perspective cross-sectional views showing aseparation chuck of an embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Methods and apparatus are described in which a free standing lamina isformed and separated from a donor body without adhesive or permanentbonding to a support element. Methods and apparatus of this inventioninclude implanting a first surface of a donor body with an ion dosageand heating the donor body to an implant temperature during implanting.The first surface of the donor body is separably contacted with a firstsurface of a susceptor assembly and a lamina is exfoliated from thedonor body by applying a thermal profile to the donor body. The laminamay then be separated from the donor body. In some embodimentsseparation methods comprise applying a deforming force to a surface ofthe lamina or donor body. Implantation and exfoliation conditions may beadjusted according to the material of the donor body in order tomaximize the defect free area of a thin free standing lamina.

Conventional photovoltaic cells formed from silicon bodies include p-ndiodes in which a depletion zone forms at the p-n junction as shown inFIG. 1. The silicon donor body used to form photovoltaic cells istypically about 200 to 250 microns thick. Thinner laminas formed fromsilicon donor bodies may be used to form photovoltaic cells by permanentfixation of the lamina to support elements either via epitaxial growth,adhesive material or other methods that result in a bonded lamina priorto cleavage or separation from a donor body. Typically, lamina formed inthis manner must either incorporate the support element into anyresultant photovoltaic cell or engage in a debonding step to remove thesupport element. In the present invention methods and apparatus aredescribed in which a thin, free standing lamina may be formed andseparated from a donor body without adhesive or permanent bonding to asupport element and without requiring debonding or cleaning steps priorto photovoltaic cell fabrication. In the present invention a donor bodyis implanted through a first surface to form a cleave plane. The firstsurface of a donor body may then be placed adjacent to a supportelement. A heating step is performed that exfoliates a lamina from thefirst surface donor body, creating a second surface. This process occursin the absence of bonded support element on the lamina. The ionimplantation and exfoliation conditions may have a significant effect onthe quality of the lamina produced by this method and may be optimizedto reduce the amount of physical defects that may be formed in the freestanding lamina. Methods for the separation of the exfoliated thin freestanding lamina are also described.

Sivaram et al., U.S. patent application Ser. No. 12/026,530, “Method toForm a Photovoltaic Cell Comprising a Thin Lamina,” filed Feb. 5, 2008,owned by the assignee of the present invention and hereby incorporatedby reference, describes the fabrication of a photovoltaic cellcomprising a thin semiconductor lamina formed of non-depositedsemiconductor material. Referring to FIG. 2A, in embodiments of Sivaramet al., a semiconductor donor body 20 is implanted through first surface10 with one or more species of gas ions, for example hydrogen and/orhelium ions. The implanted ions define a cleave plane 30 within thesemiconductor donor body. As shown in FIG. 2B, donor body 20 is affixedat first surface 10 to receiver 60. Referring to FIG. 2C, an anneal stepcauses lamina 40 to cleave from donor body 20 at cleave plane 30,creating second surface 62. In embodiments of Sivaram et al., additionalprocessing before and after the cleaving step forms a photovoltaic cellcomprising semiconductor lamina 40, which is between about 0.2 and about100 microns thick, for example between about 0.2 and about 50 microns,for example between about 1 and about 20 microns thick, in someembodiments between about 1 and about 10 microns thick or between about4 and about 20 or between about 5 and about 15 microns thick, though anythickness within the named range is possible. FIG. 2D shows thestructure inverted, with receiver 60 at the bottom, as during operationin some embodiments. Receiver 60 may be a discrete receiver elementhaving a maximum width no more than 50 percent greater than that ofdonor body 10, and preferably about the same width, as described inHerner, U.S. patent application Ser. No. 12/057,265, “Method to Form aPhotovoltaic Cell Comprising a Thin Lamina Bonded to a Discrete ReceiverElement,” filed on Mar. 27, 2008, owned by the assignee of the presentapplication and hereby incorporated by reference. Alternatively, aplurality of donor bodies may be affixed to a single, larger receiver,and a lamina cleaved from each donor body.

Using the methods of Sivaram et al., photovoltaic cells, rather thanbeing formed from sliced wafers, are formed of thin semiconductorlaminae without wasting silicon through kerf loss or by fabrication ofan unnecessarily thick cell, thus reducing cost. The same donor wafercan be reused to form multiple laminae, further reducing cost, and maybe resold after exfoliation of multiple laminae for some other use.

In the present invention, a free standing lamina is formed by implantinga semiconductor donor body with ions to define a cleave plane andexfoliating a semiconductor lamina from the donor body at the cleaveplane. The lamina has a non-bonded first surface and a non-bonded secondsurface opposite the first. After the exfoliation step, the lamina isseparated from the donor body and fabricated into a photovoltaic cell inwhich the lamina comprises a base region of the photovoltaic cell. Thethickness of the lamina may be between about 4 microns and about 20microns. One, two or more layers may be formed on the first surface ofthe lamina before incorporating the lamina into a photovoltaic cell.One, two or more layers may be formed on the second surface of the freestanding lamina. The thickness of lamina is determined by the depth ofcleave plane. In many embodiments, the thickness of lamina is betweenabout 1 and about 10 microns, for example between about 2 and about 5microns, for example about 4.5 microns. In other embodiments, thethickness of lamina is between about 4 and about 20 microns, for examplebetween about 10 and about 15 microns, for example about 11 microns. Thesecond surface is created by cleaving. While different flows arepossible, in general, the thin lamina is provided without permanent oradhesive fixing to a support element. In most embodiments, it has beenexfoliated and separated from a larger donor body, such as a wafer orboule.

Turning to FIG. 3 in which the methods of present invention areoutlined, a donor body is first implanted with ions through a firstsurface to form a cleave plane (Step 1, FIG. 3). Implant conditions maybe adjusted to mitigate the appearance physical defects (e.g., tears,cracks, rips, wave-front defects, radial striations, flaking or anycombination thereof) in the lamina ultimately formed. In one embodimentthe physical defects comprise cracks, and methods of this inventionprovide for a free standing lamina wherein the total length of thecracks is less than 100 mm. Physical defects include any defects whichmay cause shunting or reduced performance in completed cells. The areaof a lamina that comprises the physical defect may be equivalent to thearea rendered unusable in a photovoltaic cell. Implant conditions thatmay be adjusted to maximize the area that is substantially free ofdefects in the cleaved lamina include the temperature and/or thepressure applied to the donor body during implantation. In someembodiments the implant temperature may be maintained between 25 and300° C., such as between 100 and 200° C. or between 120 and 180° C. Oneaspect of the invention is that the implant temperature may be adjusteddepending upon the material and orientation of the donor body. In someembodiments, the material is {111} oriented silicon and the implanttemperature may be between 150 and 200° C. In other embodiments thematerial is {100} oriented silicon and the implant temperature may bebetween 25 and 150° C. The methods disclosed herein may apply to anyother orientation of semiconductor donor bodies as well such as {110}oriented silicon, or {001}. The implant temperature may be optimized forany silicon orientation and implant energy. Other implantationconditions that may be adjusted may include initial process parameterssuch as implant dose and the ratio of implanted ions (e.g., H:He ratio).In some embodiments implant conditions may be optimized in combinationwith exfoliation conditions such as exfoliation temperature, exfoliationsusceptor vacuum level, heating rate and/or exfoliation pressure inorder to maximize the area that is substantially free of physicaldefects present in the lamina. In some embodiments, greater than 90% ofthe surface area of the lamina produced by methods of this invention isfree from physical defects.

Following the implantation to form a cleave plane, the donor body may becontacted to a temporary support element (FIG. 3, Step 2) such as asusceptor assembly for further processing. Typically donor bodies,lamina or photovoltaic cells in various stages of manufacture may beaffixed to temporary carriers with adhesive or via chemical bonding.When adhesive is used, additional steps are required to initiate thedebonding of the lamina and/or to clean the surface of the photovoltaiccell and the temporary carrier after detachment. Alternatively, supportelements may be dissolved or otherwise removed and rendered unusable forfurther support steps. In one aspect of this invention, the donor bodyis separably contacted, without adhesive or permanent bonding, with asupport element such as a susceptor assembly in order to stabilize thelamina during exfoliation. The contact may be direct contact between thedonor body and support element, and comprise no adherents or bondingsteps that require a chemical or physical step to disrupt the contactbeyond merely lifting the donor body or lamina from the susceptor. Thesusceptor may then be reused as a support element without furtherprocessing. In some embodiments of methods of this invention, theimplanted donor body may be separably contacted with a support elementsuch as a susceptor assembly wherein the interacting force between thedonor body and the susceptor during exfoliation is solely the weight ofthe donor body on the susceptor or solely the weight of the susceptorassembly on the donor body. In the case in which contact is establishedsolely by the weight of the donor body, the donor body may be orientedwith the implanted side facing down and in contact with the susceptor.Alternatively, the donor body may be oriented with the implanted sidefacing up and not in contact with the susceptor. In this case, a coverplate may be used to stabilize the lamina during and after exfoliation.In other embodiments the contacting may further comprise a vacuum forcebetween the susceptor and the donor body. A vacuum force may be appliedto the donor body in order to temporarily fix the donor body to asusceptor assembly without the use of adhesives, chemical reactions,electrostatic pressure or the like.

Contacting the lamina to a non-bonded support element during the stepsof exfoliation and damage anneal, as in the present invention, providesseveral significant advantages. The steps of exfoliation and anneal takeplace at relatively high temperature. If a pre-formed support element isaffixed, such as with adhesives or chemicals, to the donor body beforethese high-temperature steps, it will necessarily be exposed to hightemperature along with the lamina, as will any intervening layers. Manymaterials cannot readily tolerate high temperature, and if thecoefficients of thermal expansion (CTEs) of the support element and thelamina are mismatched, heating and cooling will cause strain which maydamage the thin lamina. Thus, a non-bonded support element provides foran optimized surface for lamina manufacture independent of bonding anddebonding protocols that would potentially inhibit the formation of adefect free lamina. Annealing may occur before or after the lamina isseparated from the donor body.

Following the contacting of the donor body to the susceptor assembly,heat may be applied to the donor body to cleave a lamina from the donorbody at the cleave plane. Exfoliation conditions may be optimized tocleave the lamina from the donor body (FIG. 3, Step 3) in order tominimize physical defects in a lamina exfoliated in the absence of anadhered support element. Exfoliation parameters may be optimized withrespect to particular donor bodies. Exfoliation may occur at ambientpressure. An exfoliation thermal profile may be applied that has one, ortwo, or more thermal ramps. In some embodiments the exfoliationconditions may comprise single rapid thermal ramp to a peak exfoliationtemperature that is greater than 600° C. The thermal ramp rate may be100° C./minute, 200° C./minute or greater. The material of the susceptormay have a lower heat capacity than that of the donor body and beresistant to thermal degradation at the final exfoliation temperature inorder to facilitate exfoliation by this method. In other embodiments,the final exfoliation temperature may be between 400 and 600° C. inwhich the ramp rate is any speed, but the temperature is appliedsubstantially uniformly across the surface area of the lamina. Thesusceptor assembly may comprise a thermally anisotropic material inorder to facilitate a uniform thermal profile across the surface of thedonor body during exfoliation. In some embodiments, the donor body maybe transported into an area of higher temperature such that the heatingof the donor body progresses from one end of the body to the other in auniform manner. In one embodiment the donor body is moved from a lowertemperature zone to a higher temperature zone (e.g., a belt furnace).The rate of movement may provide for rapid changes in the temperature ofthe donor such as 60° C./minute, 200° C./minute or greater.

The exfoliated lamina may be separated from the donor body by any means(FIG. 3, Step 4) such as by applying a deforming force to a firstsurface of the donor body away from an opposite surface of the newlyformed lamina. In some embodiments, the donor body may be deformed awayfrom the exfoliated lamina. In other embodiments, the exfoliated laminamay be deformed away from the donor body. After exfoliation, the surfaceof the free standing lamina that was the first surface of the donor bodymay be separably contacted with a supporting device such as a susceptorassembly. In some embodiments the contacting force may comprise a vacuumforce between the lamina and a susceptor plate. In some embodiments thecontacting force may be merely the weight of the donor body on thelamina. A chuck plate may be adhered to the donor body on a surfaceopposite the lamina. In some embodiments the adherence may be a vacuumforce applied through a porous chuck plate to the donor body. Vacuumpressure may be applied through the chuck plate, thus adhering the donorbody to the chuck plate. The chuck plate may be coupled to a flexingdevice such as a flexing arm, or a deformable plate, or the like. Aforce applied to the flexing device may deform the donor body away fromthe lamina. The force may deform any portion of the donor body, such asan edge or other region, away from the lamina. The deformation mayseparate the donor body to a distance of greater than 1 mm away from aportion of the lamina surface, freeing an edge of the donor body forsubsequent full separation of the lamina from the donor body. Theseparated lamina may remain on the susceptor plate or be transferred toa different temporary or permanent support element for furtherprocessing. In some embodiments a permanent support may be constructedon the free standing lamina.

An aspect of this invention comprises the process of fabricating aphotovoltaic cell from a free standing lamina and begins with a donorbody of an appropriate semiconductor material. An appropriate donor bodymay be a monocrystalline silicon wafer of any practical thickness, forexample from about 200 to about 1000 microns thick. Typically the waferhas Miller indices of {100} or {111}, although other orientations may beused. In alternative embodiments, the donor wafer may be thicker;maximum thickness is limited only by practicalities of wafer handling.Alternatively, polycrystalline or multicrystalline silicon may be used,as may microcrystalline silicon, or wafers or ingots of othersemiconductor materials, including germanium, silicon germanium, orIII-V or II-VI semiconductor compounds such as GaAs, InP, etc. Othermaterial may be used, such as SiC, LiNbO₃, SrTiO₃, sapphire, and thelike. In this context the term multicrystalline typically refers tosemiconductor material having grains that are on the order of amillimeter or larger in size, while polycrystalline semiconductormaterial has smaller grains, on the order of a thousand angstroms. Thegrains of microcrystalline semiconductor material are very small, forexample 100 angstroms or so. Microcrystalline silicon, for example, maybe fully crystalline or may include these microcrystals in an amorphousmatrix. Multicrystalline or polycrystalline semiconductors areunderstood to be completely or substantially crystalline. It will beappreciated by those skilled in the art that the term “monocrystallinesilicon” as it is customarily used will not exclude silicon withoccasional flaws or impurities such as conductivity-enhancing dopants.

The process of forming monocrystalline silicon generally results incircular wafers, but the donor body may have other shapes as well. Forphotovoltaic applications, cylindrical monocrystalline ingots are oftenmachined to an octagonal cross section prior to cutting wafers. Wafersmay also be other shapes, such as square. Square wafers have theadvantage that, unlike circular or hexagonal wafers, they can be alignededge-to-edge on a photovoltaic module with minimal unused gaps betweenthem. The diameter or width of the wafer may be any standard or customsize. For simplicity this disclosure will describe the use of amonocrystalline silicon wafer as the semiconductor donor body, but itwill be understood that donor bodies of other types and materials can beused.

Ions, preferably hydrogen or a combination of hydrogen and helium, areimplanted into body through a first surface to define a cleave plane, asdescribed earlier. The overall depth of the cleave plane is determinedby several factors, including implant energy. The depth of the cleaveplane can be between about 0.2 and about 100 microns from the firstsurface, for example between about 0.5 and about 20 or about 50 microns,for example between about 1 and about 10 microns, between about 1 or 2microns and about 5 or 6 microns, or between about 4 and about 8microns. Alternatively, the depth of the cleave plane can be betweenabout 5 and about 15 microns, for example about 11 or 12 microns.

Temperature and dosage of ion implantation may be adjusted according tothe material to be implanted and the desired depth of the cleave plane,in order to provide a free standing lamina that is substantially free ofphysical defects. The ion dosage may be any dosage such as between1.0×10¹⁴ and 1.0×10¹⁸ H/cm². The implant temperature may be anytemperature such as greater than 140° C. (e. g., between 150 and 250°C.). The implant conditions may be adjusted based on the Miller indicesof the donor body and the energy of the implanted ions. For example,monocrystalline silicon with Miller indices of {111} may require adifferent set of implantation conditions than mono-crystalline silicondonor wafers with Miller indices of {100}. One aspect of the presentinvention comprises adjusting implantation conditions to maximize thearea that is substantially defect-free in a lamina. In some embodimentsthe implant dose may be less than 1.3×10¹⁷ H/cm² in combination with animplant temperature that is greater than 25° C., such as between 80° C.and 250° C. In some embodiments a monocrystalline silicon donor bodywith Miller indices of {111} may be implanted at a temperature between150 and 200° C. In some embodiments a monocrystalline silicon donor bodywith Miller indices of {100} may be implanted at a temperature between100 and 150° C. In some embodiments higher implant temperatures mayresult in more uniform exfoliation.

Referring to FIG. 4A, the implanted surface 10 of the donor body 20 maybe separably contacted with a support element such as a susceptorassembly 400. The susceptor assembly may be in contact with the donorbody while remaining unbonded to the donor body. The force of thecontact between the donor body and the susceptor assembly duringexfoliation may be solely the weight of the donor body. Alternativelythe entire assembly and donor body may be inverted and the contactingforce may be the weight of the susceptor assembly on the donor body. Insome embodiments the contacting force between the donor body and thesusceptor may be augmented by a vacuum force between the susceptor andthe donor body. The material properties of the susceptor assembly mayfacilitate the exfoliation of a substantially defect-free lamina fromthe donor body. The susceptor assembly 400 may comprise a singlesusceptor plate that is flat as in FIG. 4A. In some embodiments thesurface of the susceptor assembly may comprise a material havingsubstantially the same coefficient of thermal expansion (CTE) as thedonor body over a wide range of temperatures (e.g., 0 to 1000° C.). Thesusceptor assembly may comprise a material having a heat capacity thatis substantially the same or lower than the heat capacity of the donorbody in order to support a rapid thermal ramp to an exfoliationtemperature greater than 400° C.

In other embodiments as shown in FIG. 4B, the susceptor assembly 401 maycomprise multiple plates to provide appropriate conditions for theprocessing of a lamina from a donor body 20. In some embodiments thecontacting force between the susceptor assembly 401 and the donor bodymay be a vacuum force applied through vacuum channels 410 to a poroussusceptor plate 405 of the susceptor assembly (as depicted in FIG. 4B).When a vacuum force is utilized to hold the donor body, the susceptorplate 405 may be porous graphite or any material permeable to a vacuumpressure. For example, the material of the porous plate 405 may compriseporous graphite, porous boron nitride, porous silicon, porous siliconcarbide, laser-drilled silicon, laser-drilled silicon carbide, aluminumoxide, aluminum nitride, silicon nitride or any combination thereof.Vacuum pressures in the range of about 0 to about −100 psi, (e.g.,between 0 psi and −15 psi) may be applied. The susceptor assembly 401may comprise a first plate 405 that has a coefficient of thermalexpansion (CTE) similar or substantially the same as the coefficient ofthermal expansion of the donor body 20. In some embodiments the thermalprofile applied during exfoliation must be substantially uniform acrossthe surface of the donor body to facilitate the successful exfoliationof a free standing lamina. In order to achieve a uniform thermal profileacross the surface of the donor body, the susceptor assembly maycomprise a second plate 415 adjacent to the first plate 405, with athermal conductivity of the second plate 415 that is preferentiallyhigher in a plane parallel to the donor body compared to in a directionnormal to the donor body. A thermally anisotropic material such aspyrolytic graphite is well suited to facilitate the application of asubstantially uniform thermal profile on the donor body in this manner.The susceptor assembly may optionally comprise a thermally insulativeplate 425, such as a quartz plate disposed below the thermallyanisotropic plate 415 in order to facilitate the maintenance of thethermal profile needed for exfoliation by thermally isolating the donorbody from potentially cooling forces such as an operating vacuummanifold.

Following contact of the donor body to a susceptor assembly, a thermalexfoliation protocol may be applied that results in a free standinglamina that is substantially free of physical defects that is cleavedfrom the donor body 20 at the cleave plane 30. The exfoliation protocolmay comprise one or two or more thermal ramps to one or two or more peakexfoliation temperatures, followed by thermal soaks for periods of timesuch as less than 1, 2, 3, 4, 5 or 6 minutes. Peak exfoliationtemperatures may be between 350 and 900° C., such as between 350 and500° C. or between 500 and 900° C. Ramp rates during thermal exfoliationprofiles may also be optimized. Thermal ramp rates may range from, forexample, 0.1° C./second to 20° C./second. Exfoliation pressures may beambient pressure, or higher. The thermal exfoliation profile may beoptimized according the material and orientation of the donor body inorder to form a free standing lamina that is substantially free ofphysical defects.

In some embodiments a monocrystalline silicon lamina may be exfoliatedfrom a donor body oriented at {111} by applying an exfoliation thermalprofile comprising a single thermal ramp rate that is faster than 15°C./second to a final exfoliation temperature that is greater than 600°C. The peak exfoliation temperature may be held for 100, 50, 25 secondsor less. In other embodiments the thermal profile may comprise a ramprate of between 0.1 and 5° C./second to a peak exfoliation temperatureof between 400 and 600° C. in which the thermal ramp rate issubstantially the same across the surface area of the lamina. The peakexfoliation temperature may be held for less than 3 minutes, 1 minute,or less than 30 seconds. The susceptor may comprise a thermallyanisotropic material, such as second plate 415 of FIG. 4B, in order tofacilitate the application of a uniform thermal profile across thesurface of the donor body during exfoliation.

Alternatively, exfoliation may comprise two or more thermal ramps toprovide for a more controlled exfoliation process. Multiple thermalramps may accommodate donor bodies with Miller indices of {111}, {100}or other orientations. For example, the thermal profile may include afirst thermal ramp rate that is between 10 and 20° C./second to a peaktemperature between 350 and 500° C., followed by a second thermal ramprate that is between about 5 and 20° C./second to peak temperature thatis between 600 and 800° C. The peak exfoliation temperatures after eachthermal ramp may be held for less than 60 seconds, followed by a cooldown or further processing to anneal or separate the exfoliated lamina.In some embodiments the exfoliation protocol may comprise two or morethermal ramps under thermally anisotropic conditions to provide for amore controlled exfoliation process. Other examples of multiple thermalramp rates include a first thermal ramp between 0.5 and 10° C./second toa peak temperature that is between 350 and 450° C. followed by a secondthermal ramp between about 0.1 and 5° C./second to peak temperature thatis between 450 and 700° C. The peak exfoliation temperatures after eachthermal ramp may be held for less than10 seconds followed by a cool downor further processing to anneal the exfoliated lamina. Thermal profilesmay be applied by moving the susceptor assembly/cleaved donor body froma first zone of one temperature into a second zone of a differenttemperature. The first temperature may be lower than the secondtemperature. This process may be achieved via a belt furnace or otherconveying device.

It has been found that the step of implanting to define the cleave planemay cause damage to the crystalline lattice of the monocrystalline donorwafer. This damage, if unrepaired, may impair cell efficiency. In thepresent disclosure, annealing may remove residual physical defects inthe exfoliated lamina. A relatively high-temperature anneal, for exampleat greater than 800, 850, 900, or 950° C., will repair most implantdamage in the body of the lamina. After exfoliation, the free standinglamina may be contacted to a susceptor, with the donor body remaining ontop. The donor body may be deformed away from the exfoliated lamina byapplying a deforming force to the surface of the donor body opposite thelamina. This method may apply a sufficiently gentle force in order toseparate the donor from a lamina that is less than 50 μm thick withoutdamaging the lamina. A vacuum chuck apparatus is then placed on top ofdonor body for contact with a surface of the donor body opposite thelamina. The first chuck plate of the vacuum chuck apparatus may coverthe entire surface opposite the lamina as in FIG. 5 (chuck plate 515) ora portion of the surface opposite the lamina as in FIG. 6 (chuck plate615). The first chuck plate may be a porous plate (e.g., porousgraphite, porous boron nitride, porous silicon, porous silicon carbide,laser-drilled silicon, laser-drilled silicon carbide, aluminum oxide,aluminum nitride, silicon nitride or any combination thereof) orcomprise a vacuum channel. A vacuum is applied through the first chuckplate, vacuum chucking the donor body. Next, the first chuck plate isdeflected. A pressure may be applied to the backside of the flexingdevice, which causes a slight deflection of the flexing device,contacting plate and vacuum chucked donor body. An aspect of thesevacuum chuck methods is that the edge of the donor body pulls away fromthe lamina first, allowing air to rush in between the donor and laminasurfaces. This action eliminates suction over the newly formed surfaceof the lamina that may result in the appearance of physical defects.

Referring now to FIGS. 5A and 5B, in some embodiments the separation ofthe lamina from the donor body may occur by the deformation of the donorbody away from the lamina using a flexing plate. The deformation mayfacilitate the separation of the donor body from the free standinglamina in a manner that minimizes defects forming in the free standinglamina. FIG. 5A depicts a first step in an embodiment of this methodwherein the donor body 20 is coupled to a separation chuck 500 such as avacuum chuck. The chuck 500 may comprise a first chuck plate 515 thatmay hold to the surface 520 of donor body 20 opposite the lamina 40 viaa vacuum pressure applied through vacuum channels 525 or any otheradherent force. The first chuck plate 515 may be coupled to a flexingdevice such as a compliant arm, flexing arm, flexible plate 535 or thelike. The flexing device may be coupled to a backing plate 545 orsupport arm, pivot point or the like. The exfoliated lamina 40 may beseparably contacted to a susceptor plate 405 in a susceptor assembly402. Additional contacting force may be applied to the susceptor plate405 via vacuum pressure applied via vacuum channels 410. Separation isachieved by applying a force to the flexing device that flexes thesurface of the donor body opposite the lamina. An embodiment of thisseparation is depicted in FIG. 5B showing the flexure of the flexibleplate 535 and resultant deformation of the donor body 20 away from thelamina 40. In this embodiment, a positive pressure is applied to thebackside of the flexible plate 535 via channels 555 which causes slightdeflection of the flexible plate 535, the first chuck plate 515 and thechucked donor body 20. The positive pressure may be applied by any meanssuch as a gas flow between the flexible plate 535 and the backing plate545. A portion of the donor body 20 may be deformed between 1 and 3 mmor more from the lamina to initiate the separation of the donor bodyaway from the cleaved lamina 40 which remains stationary on thesusceptor plate 405. In an alternative embodiment, the donor body mayremain fixed to a susceptor plate, while the cleaved lamina is attachedto the chuck plate and separated from the donor body as described above.

FIGS. 6A and 6B depict an embodiment of the separation process whereinthe separation chuck comprises a first chuck plate 615 that adheres toonly a portion of the surface of the donor body 20 opposite the lamina40 and is coupled to a flexing device that is a rigid arm 635. Theadherence between first chuck plate 615 and donor body 20 may utilize avacuum force delivered through a vacuum channel 625. The first chuckplate 615 may be porous. The rigid arm 635 may be coupled to a pivotpoint 645 or any device designed to move the rigid arm away from thedonor body. The lamina 40 may be fixed to, or merely contacting, thesusceptor plate 405. Flexing the rigid arm 635 away from the lamina 40as shown in FIG. 6B results in the deformation of a portion of donorbody 20 away from the lamina 40 which remains stationary on thesusceptor plate 405. In an alternative embodiment, the donor body mayremain fixed to a susceptor plate, while the cleaved lamina is attachedto the chuck plate 615 and separated from the donor body as describedabove. An anneal step may be performed at any stage in the process, suchas after the separation of the free standing lamina, in order to repairdamage caused to the crystal lattice throughout the body of laminaduring the implant, exfoliation steps or separation steps. Annealing maybe performed while the lamina remains in place on the susceptorassembly, for example, at temperatures greater than 500° C., for exampleat 550, 600, 650, 700, 800, 850° C. or greater, such as about 950° C. orgreater for any amount of time. The structure may be annealed, forexample, at about 650° C. for about 45 minutes or at about 800° C. forabout ten minutes, or at about 950° C. for 120 seconds or less. In manyembodiments the temperature exceeds 850° C. for at least 60 seconds. Insome embodiments, it may be advantageous to remove the donor body beforeannealing the lamina to temperatures above 700° C. so that the structureand electronic properties of the donor are preserved for subsequentiteration of the implant-exfoliation process.

A photovoltaic device may be fabricated from the free standing thinlamina after the lamina has been annealed. The lamina may be transferredto a temporary or permanent support for further processing to this endas described in U.S. patent application Ser. No. 12/980,424, “A Methodto Form a Device by Constructing a Support Element on a ThinSemiconductor Lamina” filed on Dec. 29, 2010, and hereby incorporated byreference. This may be done, for example, using a vacuum paddle (notshown). To affect this transfer, a vacuum paddle may be placed on asecond surface, while a vacuum on a first surface is released. Followingtransfer to the vacuum paddle, the second surface is held by vacuum,while first surface is exposed. Referring to FIG. 7A, the lamina 40 maybe affixed to a temporary carrier 50, for example using an adhesive.This adhesive must tolerate moderate temperatures (up to about 200° C.)and must be readily releasable. Suitable adhesives include, for example,polyester with maleic anhydride and rosin, which is hydrocarbon-soluble;or polyisobutylene and rosin, which is detergent soluble. Temporarycarrier 50 may be any suitable material, for example glass, metal,polymer, silicon, etc. Following transfer, first surface 10 will be heldto temporary carrier 50 by adhesive, while second surface 62 is exposed.

As depicted in FIG. 7B, further processing to form a photovoltaic devicemay follow. An etch step to remove damage caused by exfoliation may beperformed, for example by applying a mix of hydrofluoric (HF) acid andnitric acid, or using KOH. It may be found that annealing is sufficientto remove all or nearly all damage and this etch step is unnecessary.The surface may be cleaned of organic materials and residual oxide,using a dilute HF solution; for example, 10:1 HF for two minutes.Following this wet process, an amorphous silicon layer 72 is depositedon second surface 62. This layer 72 may be heavily doped silicon and mayhave a thickness, for example, between about 50 and about 350 angstroms.FIG. 7B shows an embodiment that includes an intrinsic or nearlyintrinsic amorphous silicon layer 74 between second surface 62 and dopedlayer 72, and in immediate contact with both. In other embodiments,layer 74 may be omitted. In this example, heavily doped silicon layer 72is heavily doped n-type, the same conductivity type as lightly dopedn-type lamina 40. Lightly doped n-type lamina 40 comprises the baseregion of the photovoltaic cell to be formed, and heavily dopedamorphous silicon layer 72 provides electrical contact to the baseregion. If included, layer 74 is sufficiently thin that it does notimpede electrical connection between lamina 40 and heavily doped siliconlayer 72.

A transparent conductive oxide (TCO) layer 110 is formed on and inimmediate contact with amorphous silicon layer 74. Appropriate materialsfor TCO 110 include indium tin oxide and aluminum-doped zinc oxide. Thislayer may be, for example, about between about 500 to about 1500angstroms thick, for example about 750 angstroms thick. This thicknesswill enhance reflection from a reflective layer to be deposited. In someembodiments this layer may be substantially thinner, for example about100 to about 200 angstroms. Amorphous silicon layer 76 may be applied tothe second surface after the annealing of the lamina as well.

As will be seen in the completed device shown in FIG. 7C, incident lightwill enter lamina 40 at first surface 10. After passing through lamina40, light that has not been absorbed will exit lamina 40 at secondsurface 62, then pass through TCO layer 110. A reflective layer 12formed on TCO layer 110 will reflect this light back into the cell for asecond opportunity to be absorbed, improving efficiency. A conductive,reflective metal may be used for reflective layer 12. Various layers orstacks may be used. In one embodiment, reflective layer 12 is formed bydepositing a very thin layer of chromium, for example about 30 or 50angstroms to about 100 angstroms, on TCO layer 110, followed by about1,000 to about 3,000 angstroms of silver. In an alternative embodiment,not pictured, reflective layer 12 may be aluminum, having a thickness ofabout 1000 to about 3000 angstroms. In the next step, a layer will beformed by plating. Conventional plating cannot be performed onto analuminum layer, so if aluminum is used for reflective layer 12, anadditional layer or layers must be added to provide a seed layer forplating. In one embodiment, for example, a layer of titanium, forexample between about 200 and about 300 angstroms thick, followed by aseed layer, for example of cobalt, which may have any suitablethickness, for example about 500 angstroms.

Metal support element 60 is formed on reflective layer 12 (achromium/silver stack in this embodiment). In some embodiments the metalsupport element 60 is formed by electroplating. Temporary carrier 50 andlamina 40, and associated layers, are immersed in an electrolyte bath.An electrode is attached to reflective layer 12, and a current passedthrough the electrolyte. Ions from the electrolyte bath build up onreflective layer 12, to form a continuous metal support element 60.Metal support element 60 may be, for example, an alloy of nickel andiron. Iron is cheaper, while the coefficient of thermal expansion ofnickel is better matched to that of silicon, reducing stress duringlater steps. The thickness of metal support element 60 may be asdesired. Metal support element 60 should be thick enough to providestructural support for the photovoltaic cell to be formed. A thickersupport element 60 is less prone to bowing. In contrast, minimizingthickness reduces cost. One skilled in the art will select a suitablethickness and iron:nickel ratio to balance these concerns. The thicknessof metal support element 60 may be, for example, between about 25 andabout 100 microns, for example about 50 microns. In some embodiments,the iron-nickel alloy is between about 55 and about 65 percent iron, forexample 60 percent iron.

Lightly doped n-type lamina 40 comprises the base of the photovoltaiccell, and heavily doped p-type amorphous silicon layer 76 serves as theemitter of the cell. Heavily doped n-type amorphous silicon layer 72will provide good electrical contact to the base region of the cell.Electrical contact must be made to both faces of the cell. Contact tothe amorphous silicon layer 76 is made by gridlines 57, by way of TCOlayer 112. Metal support element 60 is conductive and is in electricalcontact with base contact 72 by way of conductive layer 12 and TCO layer110.

FIG. 7C shows completed photovoltaic assembly 80, which includes aphotovoltaic cell and metal support element 60. In alternativeembodiments, by changing the dopants used, heavily doped amorphoussilicon layer 72 may serve as the emitter, while heavily doped amorphoussilicon layer 76 serves as a contact to the base region. Amorphoussilicon layers 72 and 76 may be in immediate contact with the first andsecond surfaces of the free standing lamina respectively. Incident light(indicated by arrows) falls on TCO 112, enters the cell at heavily dopedp-type amorphous silicon layer 76, enters lamina 40 at first surface 10,and travels through lamina 40. Reflective layer 12 will serve to reflectsome light back into the cell. In this embodiment, receiver element 60serves as a substrate. Receiver element 60 and lamina 40, and associatedlayers, form a photovoltaic assembly 80. Multiple photovoltaicassemblies 80 can be formed and affixed to a supporting substrate 90 or,alternatively, a supporting superstrate (not shown). Each photovoltaicassembly 80 includes a photovoltaic cell. The photovoltaic cells of amodule are generally electrically connected in series.

Susceptor Apparatus

Referring now to FIG. 8A and 8B, the susceptor assembly as previouslydescribed in FIG. 4A and 4B may comprise one or more susceptor plates.The susceptor assembly 400 may be set in the lower part of a susceptorchamber 800 shown in FIG. 8B and be configured to support appropriateconditions for exfoliating, annealing or separating a free standinglamina. In FIG. 8A, a first plate 405 may be used for contacting a firstsurface of the donor body and provide separable support for the laminaduring exfoliation, separation, annealing or any combination thereof.The first susceptor plate 405 may be used throughout the laminaproduction process, or separate plates with separate propertiesoptimized for particular steps may be used. For example, the donor bodymay be contacted with a first susceptor plate assembly duringimplantation, a second susceptor plate during exfoliation and a thirdsusceptor plate during separation. An optional upper surface (e.g., achuck, not shown) may be utilized for contact with a second surface ofthe donor body opposite the first surface. The susceptor assembly 400provides physical support for the thin lamina after exfoliation and alsomay provide thermal characteristics conducive to the exfoliation andannealing protocols utilized. In some embodiments the first susceptorplate 405 may be an inert solid such as graphite. In some embodiments ofthe present invention, the donor body or lamina is separably contactedto a susceptor assembly that is vacuum permeable. A porous material maybe utilized for the first susceptor plate 405 to enable vacuum pressureto hold the donor body or the lamina to the susceptor duringexfoliation. Porous materials may comprise porous graphite, porous boronnitride, porous silicon, porous silicon carbide, laser-drilled silicon,laser-drilled silicon carbide, aluminum oxide, aluminum nitride, siliconnitride or any combination thereof.

The vacuum may be achieved by applying negative gauge pressure in thesurrounding environment (e.g., air or nitrogen) or by directed vacuumpressure via a series of vacuum channels 410. Selection of a poroussusceptor plate material that is conducive to the process flow isimportant to the exfoliation process. Material properties that areconducive to the exfoliation process include: a low coefficient ofstatic friction (CSF with values, e.g., 0.1-0.5), low hardness (<10 onMohs Scale of Hardness), average pore diameter less than approximately15 micrometers, ability to machine flatten (that is, able to useconventional machinery techniques/materials on these susceptors), lowroughness (<1 μm roughness), flatness (<10 μm waviness over body),sufficient electrical conductivity to prevent the occurrence of staticcharges between the lamina and the susceptor, etc. In one embodiment thefirst susceptor plate 405 may have a coefficient of thermal expansion(CTE) that is substantially the same as the CTE of the donor body. Inother embodiments the susceptor plate may have a heat capacity that isthe same or lower than the heat capacity of the donor body. In someembodiments the donor body is monocrystalline silicon and the heatcapacity of the susceptor is about the same as that of the silicon(about 19.8 J/mol-° K).

With these constraints, many engineering ceramics and other materialsmay be selected to provide these characteristics for the first susceptorplate 405. In one embodiment, Ringsdorff™ graphite grade R6340 may beused because it has a CTE similar to that of silicon. This is importantin order to prevent lateral forces from being applied to the donor bodyor lamina during thermal treatments associated with exfoliation orannealing. With graphite whose CTE is not similar to that of silicon,these temperature changes may result in wrinkling or tearing of thelamina. With the CTE-matched graphite, the lamina may remain under lightretention vacuum or no vacuum retention during these temperaturechanges. A bulk etch may be applied to the graphite to improve purity. Acommon bulk etch process consists of a 24 hour high temperature bake-outin a vacuum chamber that has been introduces with chloride gas.

In other embodiments of the exfoliation process, a rapid, hightemperature thermal profile is applied. In these embodiments a susceptorplate that is resistant to off-gassing or degrading at temperatures ashigh as 800 or 900 or 1000° C. is desired. The susceptor material mayhave characteristics to prevent contamination of the donor body, such asbeing able to withstand the temperatures and atmospheric exposures ofthe processes without undergoing material degradation. The material maybe intrinsically resistant to degradation or coated with a material thatacts as a barrier to donor body contamination at elevated temperatures.For example porous silicon carbide—which is hard, durable, and has goodCTE matching—may be coated with Boron Nitride, which is soft and has lowCSF and high purity. In other embodiments, optimizingporous/laser-drilled materials may be utilized. Laser-drilled materialsallow for differentiating between the necessities of the bulk of thechuck (porosity, CTE, flatness/machinability) and the necessities of thesurface material (low CSF, soft, high purity, etc.). For example, amaterial which provides an interface with the desired properties listedabove can be coated on a stock material having properties which aredesirable for the base bulk. In other embodiments, metal oxides,carbides, nitrides, ceramics, and high temperature alloys which meet thepreviously mentioned specifications are candidates for use. Thecharacteristics of the susceptor material described above beneficiallyimprove the quality of the lamina produced, including the mechanicalproperties, uniformity, and purity of the lamina.

In another embodiment of the present invention, a uniform temperatureprofile may be applied to the donor body during exfoliation. In FIG. 8A,a second susceptor plate 415 that is thermally anisotropic may bedisposed adjacent to the first plate 405 to provide a thermalconductivity that is preferentially higher in a plane parallel to thedonor body compared to in a direction normal to the donor body in orderto facilitate the application of a uniform thermal profile. Theuniformity of the exfoliation temperature profile may be increased bythe presence of pyrolytic carbon—a graphite material which is highlyconductive across the cleave plane compared to perpendicular to thecleave plane, thus being an ideal planar thermal conductor. Thethermally anisotropic second plate may comprise vacuum channels 410 inorder to facilitate the distribution of vacuum pressure to the bottomside of the first susceptor plate 405. Extra features may include vacuumchannels 455 that are machined into surface of the susceptor plate toimprove the distribution of the vacuum pressure. In the embodiment shownin FIG. 9A a second susceptor plate 915 with a set of vacuum channels955 that are shown as concentric rings connected by radial paths may beused to distribute vacuum pressure to a separate porous susceptor plate.Vacuum channels on the periphery 925 of the second susceptor plate 915may be used to distribute vacuum pressure around the periphery of thesusceptor plate in order to fix one susceptor plate to another device orplate.

In some embodiments, a heating source for the susceptor apparatus may beprovided, such as by embedding heat lamps within a susceptor chamber.The heating source may be any source capable of providing thetemperatures required for implantation, exfoliation or anneal, such asup to 1000° C. In other embodiments, the heating source may be placedseparate from the susceptor chamber, such as, but not limited to, aquartz heating or induction heating element disposed within thesusceptor chamber to heat the susceptor assembly and/or donor body.

In a further embodiment, differential vacuum channels may be used on thesusceptor plates such that the vacuum to fix the plates 405 and 415together is separate from the vacuum to hold the donor body to thesusceptor assembly 400. FIG. 8A illustrates an exemplary susceptorassembly with differential vacuum channels. In order to vary the holdingforce, the vacuum pulling through the susceptor may need to bethrottled. To decouple the effects of pulling through the porousmaterial (e.g., graphite) onto the lamina versus pulling on the firstsusceptor plate itself, differential vacuum channels for the two may beemployed. The first set of channels 410 is centrally located, and thesecontrol the suction on the lamina itself. The second set of vacuumchannels 460 and an annulus 470 is located around the edge of the firstand second susceptors—which holds the first susceptor in place,regardless of the chucking on the donor body in the center. By employingthis system it is possible to remove the lamina while still keeping thesusceptor assembly together.

In some embodiments a vacuum force is applied to fix the lamina to thesusceptor assembly during an annealing or exfoliation process which maycontribute to a cooling of the susceptor assembly. In order to achievethe high temperatures needed for the annealing process or exfoliationprocess, the susceptor assembly may comprise a plate that provides athermal break between the donor body and a lower vacuum manifold. Athird susceptor plate 475 that acts as a thermal break may be added tothe susceptor assembly 400 of FIG. 8A between a vacuum manifold (notshown) and either the first 405 or second 415 susceptor plate. In analternative embodiment the first or second susceptor plate may act as athermal break between the vacuum manifold and the lamina. In someembodiments of the present invention, the thermal break in anneal and/orexfoliation may be achieved by quartz discs, such as the disc 975, shownin FIG. 9B. The number of discs may be, for example, one or twodepending on the temperature range and uniformity desired. Instead ofquartz, other thermally insulative materials capable of withstandingannealing temperatures may be used, such as a high temperature ceramic.The quartz discs are machined to enable vacuum to pass through them,while still segregating the inner and outer annuli of the differentialvacuum channels. This thermal break disc may be critical when using avacuum manifold below the susceptor assembly that is water-cooled. Thethermal break may prevent heat loss from the first susceptor plate 405,which may potentially facilitate achieving the temperatures needed toreach anneal and/or exfoliation. Properties of the thermal breaksusceptor plate 475 that may facilitate the annealing process include:low content of highly diffusive foreign materials (less than 20 PPMimpurities), coefficient of thermal expansion similar to silicon (e.g.,within 20% of silicon CTE), and high temperature compatibility (e.g.,1,000° C.), and low electrical resistivity.

Note that while the susceptor assembly with differential channelsdepicted in FIG. 8A is shown in conjunction with the thermal stack, thedifferential channel 460/470 and thermal stack 475 features may be usedindependently of each other. Similarly, the thermal stack may beutilized in any situation in which the top surface supporting the laminaoperates at a different temperature than components beneath the thermalbreak element. Furthermore, the individual elements of the thermal stack(quartz as a thermal break separating the top surface from the lowersurface—at a different temperature) may be used individually, in adifferent order, or in a different configuration.

Separation Apparatus

In FIG. 10A and 10B, an embodiment of a separation chuck 100 forseparating the donor body from the lamina in the method of FIG. 5B isshown. In operation, the surface of the donor body opposite the laminawould be placed against the separation chuck, and the exfoliated (butnot yet separated) lamina body would be placed on a susceptor assembly.Alternatively the donor body/lamina may be inverted in this apparatus.The separation chuck 100 of FIG. 10A and B involves a stack of platesincluding a porous plate 115 (for example, graphite), a flexing devicesuch as a flexible plate 135 (for example, aluminum or PEEK), and astiff support plate 145 (such as aluminum). The porous plate 115 shallbe referred to as graphite in this disclosure; however, other materialsare also possible as shall be described later. The stiff plate 145 hasdistribution channels 150 in it to apply positive pressure to thebackside of the flexible plate 135. The distribution channels may beconfigured, for example, as concentric rings connected by radialchannels, or as a linear grid. The flexible plate 135 may be secured tothe stiff plate 145 around its circumference. When a positive pressureis applied, the central portion of the flexible plate 135 will deflectinto a convex shape, such as in FIG. 10B, forcing the porous plate 115to follow that shape. The flexible plate may deflect by, for example, onthe order of 1 or 2 or more millimeters. The operating pressure of thedevice may be any pressure, for example, 0.1-5 bar. This pressuredepends on the thickness of the materials in the device. Threerequirements of the flexible plate are mechanical strength to takepressure applied to it, compliance to elastically bend (as opposed tobreaking) and being impermeable to pressurized air. In one embodiment,the porous layer 115 is approximately 3 mm in thickness, and theimpermeable flexible plate 135 is approximately 5 mm thick. Othermaterial choices for the flexible layer 135 include pliable metals likealuminum, thin gauge steel, or polymer, elastomeric or rubber-basedmaterials. In some embodiments the donor body (not shown) may be heldagainst the porous plate 115 by applying a vacuum between the flexibleplate and porous plate. Since the graphite plate is porous, vacuum maybe applied to distribution channels 160 behind the plate 115, via avacuum inlet to the vacuum volume, to provide an even suction throughthe porous plate 115.

EXAMPLES

Lamina Dormation from a {111} Monocrystalline Donor Wafer

The process begins with donor wafer with Miller indices of {111}. Afirst surface is provided that is substantially planar, but may havesome preexisting texture. The donor body was implanted with a total iondosage of 4.0—10¹⁶ H atoms/cm³ at 400 keV. The implant temperature wasapproximately ° C. The implantation resulted in a cleave plane 4.5 μmfrom a first surface of the donor body. The donor body was doped with ann-type dopant such as boron to a resistivity between 1 and 3 ohm-cm.

After implantation the implanted surface of the donor wafer wascontacted with a susceptor assembly. The susceptor assembly comprised asusceptor plate of porous graphite. Additionally, the porous graphitehad been machine smoothed with 1500 grit sandpaper to provide auniformly flat and smooth surface. No vacuum pressure was applied to thewafer. Once contacted to the susceptor assembly, a thermal exfoliationprofile was applied that comprised two thermal ramps. Beginning at roomtemperature the following ramp sequence was applied: 15° C./s ramp to400° C., hold at 400° C. for 60 seconds, followed by a 10° C./s ramp to700° C. At this point the lamina had exfoliated from the donor wafer andwas annealed by ramping to 950° C. at 10° C./s and holding for 1 minute.The wafer was then allowed to cool down to room temperature.

The donor body was separated from the lamina at room temperature whilethe first surface of the lamina (formerly the donor body) remained fixedto the susceptor plate. A vacuum force of −13 psi was applied to aporous plate in a susceptor assembly, fixing the lamina to the susceptorassembly. A portion of the second surface of the donor body opposite thefirst surface was contacted to a porous plate of a separation chuckcoupled to a vacuum line. The porous plate of the separation chuck wascoupled to a rigid arm comprising a pivot point. When a vacuum wasapplied to the porous plate of the separation chuck, a portion of theplate compacted against the donor body causing the rigid arm to pivot onthe pivot point, lifting a portion of the donor body away from thelamina. After the initial separation from the lamina, the donor body waslifted manually from the lamina and returned to the process line. Thelamina was further processed to form a photovoltaic device. Theseparation process occurred at ambient temperature and pressure.

Lamina from a {100} Monocrystalline Donor Wafer

The process begins with donor wafer with Miller indices of {100}. Afirst surface is provided that is substantially planar, but may havesome preexisting texture. The donor body was implanted with a total iondosage of 8.0×10¹⁶ H atoms/cm³ at 400 keV. The implant temperature wasapproximately 160° C. The implantation resulted in a cleave plane 4.5 μmfrom a first surface of the donor body.

After implantation, the implanted surface of the donor wafer wascontacted with a susceptor assembly. The susceptor assembly comprised asusceptor plate of porous graphite. The porous graphite had been machinesmoothed with 1500 grit sandpaper to provide a uniformly flat smoothsurface. The susceptor assembly further comprised a second susceptorplate that was thermally anisotropic. The second susceptor platecomprised pyrolytic graphite and provided a thermally anisotropicmaterial to facilitate a uniform thermal treatment. The donor body wasfixed to the susceptor assembly by the application of −13 psi vacuum tothe first susceptor plate.

After contacting the susceptor assembly to the donor body, a thermalexfoliation profile was applied that comprised a thermal ramp rate of2.3° C./s to a first exfoliation temperature of 440° C. which was heldfor 60 seconds, followed by a thermal ramp rate of 0.2° C./s to 490° C.,which was held for 500 seconds. After exfoliation, a thin free standinglamina comprising a first surface that was the first surface of thedonor body and a second surface opposite the first surface was annealedat 950° C. for 3 minutes. The wafer was allowed to cool to roomtemperature.

The donor body was separated from the lamina at room temperature whilethe first surface of the lamina (formerly the donor body) remained fixedto the susceptor plate with an applied vacuum force of −13 psi. Aportion of second surface of the donor body opposite the first surfaceof lamina was contacted to the porous plate of a separation chuck thatwas coupled to a vacuum line. The porous plate was also coupled to arigid arm comprising a pivot point. When a vacuum was applied to theporous plate, a portion of the porous plate compacted against the donorbody causing the rigid arm to pivot on the pivot point, lifting aportion of the donor body away from the lamina. The donor body waslifted manually from the donor body and returned to the process line.The lamina was further processed to form a photovoltaic device.

A variety of embodiments have been provided for clarity andcompleteness. Clearly it is impractical to list all possibleembodiments. Other embodiments of the invention will be apparent to oneof ordinary skill in the art when informed by the present specification.Detailed methods of fabrication have been described herein, but anyother methods that form the same structures can be used while theresults fall within the scope of the invention. The foregoing detaileddescription has described only a few of the many forms that thisinvention can take. For this reason, this detailed description isintended by way of illustration, and not by way of limitation. It isonly the following claims, including all equivalents, which are intendedto define the scope of this invention.

1. A method of producing a lamina from a donor body comprising: a.implanting a first surface of a donor body with an ion dosage to form acleave plane; b. separably contacting the donor body to a first surfaceof a susceptor assembly, wherein the donor body and the first surface ofthe susceptor assembly are in direct contact; c. exfoliating a laminafrom the donor body at the cleave plane, wherein the first surface ofthe donor body comprises a first surface of the lamina; and d. deformingthe first surface of the lamina or a second surface of the donor body toseparate the lamina from the donor body, by applying a deforming forceto the first surface of the lamina or the second surface of the donorbody, wherein the second surface of the donor body is opposite of thefirst surface of the donor body.
 2. The method of claim 1 whereindeforming the second surface of the donor body comprises: a. coupling afirst chuck plate to the second surface of the donor body, wherein thechuck plate is coupled to a flexing device; and b. applying thedeforming force to the flexing device, wherein the deforming forcedeforms the flexing device and the first chuck plate and the donor bodyaway from the lamina.
 3. The method of claim 1 wherein deforming thefirst surface of the lamina comprises: a. coupling a first chuck plateto the first surface of the lamina, wherein the chuck plate is coupledto a flexing device; and b. applying the deforming force to the flexingdevice, wherein the deforming force deforms the flexing device and thefirst chuck plate and the lamina away from the donor body.
 4. The methodof claims 2 and 3 wherein the first chuck plate comprises a porousmaterial through which a vacuum pressure can permeate, and wherein themethod further comprises the step of applying a vacuum pressure betweenthe first chuck plate and the donor body, wherein the vacuum pressureenables the coupling of the donor body to the first chuck plate.
 5. Themethod of claim 4 wherein the porous material is selected from the groupconsisting of porous graphite, porous boron nitride, porous silicon,porous silicon carbide, laser-drilled silicon, laser-drilled siliconcarbide, aluminum oxide, aluminum nitride, and silicon nitride.
 6. Themethod of claims 2 and 3 further comprising a backing plate attached tothe periphery of the flexing device, and wherein the step of applyingthe deforming force comprises forming a pressure volume between theflexing device and the backing plate.
 7. The method of claim 1 whereindeforming the donor body comprises displacing a portion of the donorbody between 1 and 3 mm from the first surface of the lamina.
 8. Themethod of claim 2 further comprising the step of transferring the laminafrom the susceptor assembly to a transfer chuck, wherein the transferchuck comprises a porous transfer plate through which vacuum pressurecan permeate, and wherein a second surface of the lamina is separablycontacted with a first surface of the porous transfer plate.
 9. Themethod of claim 1 wherein the exfoliation occurs at ambient pressure.10. The method of claim 1 further comprising the step of reusing thesusceptor assembly after separating the lamina from the donor body. 11.The method of claim 1 wherein the first surface of the susceptorassembly comprises a first plate capable of contacting with the donorbody, wherein the first plate comprises a porous material through whichvacuum pressure can permeate.
 12. The method of claim 11 wherein thefirst plate comprises porous graphite, porous boron nitride, poroussilicon, porous silicon carbide, laser-drilled silicon, laser-drilledsilicon carbide, aluminum oxide, aluminum nitride, or silicon nitride orany combination thereof.
 13. The method of claim 11 wherein the firstplate has a first coefficient of thermal expansion and the donor bodyhas a second coefficient of thermal expansion, and wherein the first andsecond coefficients of thermal expansion are substantially the same. 14.The method of claim 11 wherein the susceptor assembly further comprisesa second plate adjacent to the first plate, and wherein the second plateis thermally anisotropic.
 15. The method of claim 14 wherein the secondplate comprises pyrolytic graphite.
 16. The method of claim 11 whereinthe first plate comprises a material that has a heat capacity that islower than the heat capacity of the donor body.